One of the most widely used methods of printing uses a screen. The basis of the screen is a woven thread width a net-like structure of holes. Early screens were fabricated from silk, but partly because of the expense of this material and partly from the point of view of performance, silk has been replaced by nylon, polyester or even metal. The screen is selectively blocked so that ink will pass through only in areas which are necessary to print. Screens come in a variety of mesh openings and thicknesses, depending on the particular application. The screen serves as a support for the blocking material, supporting portions of the blocking material which are unconnected and would otherwise be unsupported. For instance, in printing the letter ‘O’, in order to allow ink to form the outside of the letter without filling the center, there must be blocking material in the center of the letter. The screen serves to support this center blocking material so that it cannot fall out of the stencil. The screen is tensioned on a frame and ink is pressed onto the stencil with a squeegee so that it is applied to a substrate placed below the screen in those places where it is necessary to print.
The seen printing process is used with inks formulated to adhere to a large variety of surfaces, and the printing process itself can handle a large variety of shapes. This makes screen printing the most versatile of printing processes and it is widely used to print on textiles, packaging, china-ware, glass, plastics, wood and metals, printed circuit boards and posters.
In order to make a screen, it is necessary to prepare artwork, then to photograph it to produce a positive film that in turn can be used to produce the exposed pattern on the photosensitive screen coating. Such original artwork may now be most easily prepared using a computer. The digital information in the computer is then used in an image-setter to produce the positive film. There are a variety of prior art photographic methods of producing the stencil.
In the direct method, solutions of light-sensitive coatings are applied directly onto the screen, then dried and hardened into a printing pattern by exposure to ultraviolet (UV) light through a positive film in which the image areas are opaque to UV light. After exposure, the unexposed, unhardened coating is washed away allowing ink to pass through.
There are other, indirect methods of producing the stencil. The light sensitive coating may be prepared as a pre-sensitized film on an intermediate base. The film can then either be transferred onto the screen before exposure and development or after exposure and development.
There is a growing need in many markets to print low run lengths and print on demand. This is because it is expensive to carry large stocks of pre-printed items and because there is an increasing demand for product customization to the need of individual customers or to relatively small groups of customers instead of mass production. The speed and cost of screen production becomes important and any means of simplifying and reducing costs is advantageous. There is also a trend to use computers to prepare artwork for printing and it would obviously be more convenient if the screen could be prepared directly from the computer information without recourse to the preparation of an intermediate photomask.
Inventors have attempted to use inkjet to produce masks on the screen itself, so that the unimaged, unprotected parts of the screen can be flood-cured by UV radiation. An example of this is described in EP 0 492 351 B1 by Gerber Scientific Products Inc. The problems of ink receptivity of the screen are acknowledged and the preferred method of overcoming these problems is by the introduction of talc onto the surface of the screen, to absorb the ink-jetted ink. This absorbs the liquid medium of the ink, to give a dried graphic. The graphic is described as preferably formed from a water-soluble ink.
Subsequently, similar inkjet integral masks have also been used to produce flexo plates. WO 97/25206 (Polyfibron) describes such a method. The inks used are either solvent based or “phase change”. After deposition, the ink is dried by evaporation of the volatile solvent or, in the case of phase change inks, by solidification. After the ink mask is used by flood-exposing the plate with UV light, the image areas are washed out. No mention is made of any difficulties in removing the dry mask after flood exposure, save to point out that inks are useful so long as they can be removed by subsequent washing, without damaging the surface of the plate.
The following later patents recognize the problem of post flooding washout and try to deal with it PCT WO 98/51750 (Markem Corporation) describes such a process. The inks used are “phase change”—known also as hot-melt inks. The ink dries by solidification as it impacts the screen. The patent recognizes the difficulty of removing the solid ink after it had served its purpose as a mask and the inks are formulated to be auto-dispersible in water.
GB 2 315 076 (Sericol) recognizes the same problem when phase change inks are used as integral masks for screen printing. Their solution is to use a water-soluble material having a wax-like texture.
U.S. Pat. No. 5,878,076 (McCue) attempts to circumvent the problem of mask removal after UV flood exposure by depositing only the screen itself by, for instance, inkjet, so that the deposit is in all areas except those of the image. The deposit is then subsequently flood UV-cured from both sides. As a layer of inkjet ink is relatively thin, the patent provides the possibility of multiple passes to achieve the desired screen thickness.
Therefore, it would be desirable to provide a method for screen printing which would not require the production of an intermediate positive film, would allow screen masters to work flow of the printing process, would provide an easily washable ink for forming the mask and would be quicker and more economical to use.